Adhesive strip



June 23, 1970 E. w. JANSSEN ET A1. 3,516,852

ADHESIVE STRIP Filed Oct. 2, 196'? 2 Sheets-Sheet l PULL STRIP T0 OPEN PULL STRIPv TO OPEN www0/P5 June 23, 1970 E. W. JANSSEN ET AL 3,515,852

ADHESIVE STRIP Filed Oct. 2, 1967 2 Sheets-Sheet 2 JE/@UME f4. ME75/@MK United States Patent O U.S. Cl. 117-122 7 Claims ABSTRACT OF THE DISCLOSURE Adhesive strip providing an easy-opening feature for a metal can in that the adhesive strip hermetically seals the can but is easily peeled away with the fingertips to open the can. The key to the invention is the adhesive which both continuously resists a dead load shear such as is exerted by the vapors in a can of coffee and also provides a peel resistance within the desired range of 21/2 to 61/2 pounds per 1/z-inch width.

This application is a continuation-in-part of our cpending application Ser. No. 384,447 iiled July 22, 1964 and now abandoned.

The present invention relates to the art of hermetically sealing metal cans containing coffee or other perishable commodities.

BACKGROUND OF THE INVENTION Heretofore the most common hermetically sealed metal can has been the so-called sanitary can. This requires a can opener, but even in this day of mechanization, at least half the households in the United States do not possess an efficient can opener, and even the best can opener tends to drop metal shavings into the container. Oftentimes the top falls into the can, and its removal is quite a delicate task without spilling the contents or cutting ones fingers. If the can opener breaks or is misplaced, it is virtually impossible to open the can.

Also heretofore others have fitted cans with scored tear strips to `be unwound using a key, which is commonly fastened to the lid. At best the key is hard to turn. Then, if the strip starts to slip oif the key, it is often necessary to resort to pliers. If the key is lost, the problem of opening the can may completely baffle the housewife.

Also heretofore others have attempted to make hermetically sealed cans for coffee or the like to be opened by peeling away an adhesive strip. Balocca et al. Pat. No. 3,231,078 and Schneider et al. Pat. No. 3,186,581 disclose such attempts. The Balocca patent contains no working example; and when one makes a can exactly as described in the Schneider patent and then tries to peel away the adhesive strip, the aluminum foil separates from the adhesive layer which remains on the can and must be slit to permit removal of the lid. If instead of clean tin plate the can has a decorative lithographed coating, the Schneider adhesive strip adheres very poorly, presumably due to lubricant commonly used in such coatings to prevent souffing in the can-forming equipment.

The Schneider patent acknowledges that the adhesive strip may not peel cleanly from the can if the blank is overheated while applying the adhesive strip, but if the temperature is sufficiently low to guard against this, the adhesive strip may not provide a secure hermetic seal. The Schneider patent notwithstanding, we believe that no one prior to the present invention was able to achieve a hermetic seal with an adhesive strip which would withstand 3,516,852 Patented June 23, 1970 ICC the pressure of vapors from coffee during storage and still could be easily and satisfactorily peeled away with the fingertips.

c For canning liquids, others have formed can ends with dlspensing openings sealed with peelable adhesive strips, as in Houghtelling Pat. No. 2,870,935. However, we believe that no one prior to our invention had devised an adhesive strip which would effectively seal the dispensing openings against all ordinary storage conditions and against the shocks to which the cans may be subjected in shipment and handling-and still peel away cleanly by hand with ease. While the housewife is willing to exert slightly more force to remove a short tab on an end cap than she is to remove a strip extending completely around the can, she is likely to become impatient with any closure which requires considerable exertion.

OBJECTIVES OF THE INVENTION A primary purpose of this invention is to provide an adhesive strip for a hermetically sealed metal can for coffee or the like permitting the housewife to open the can with her fingertips simply by peeling the strip away. Another object of the invention is to provide a peelable adhesive strip for opening hermetically sealed cans which will withstand can-forming operations. For example, a socalled collar can may be made from a can blank which is formed with a slit to be sealed with a peelable adhesive strip before the blank is formed into a can. This requires the peelable strip to withstand the stresses of forming and capping the blank to provide a hermetically sealed can, and the assembled can must withstand the effects of varied atmospheric conditions to which it might be exposed. Then the adhesive strip should be easily peeled away by hand at virtually any ambient temperature, at least from 40 to F., and preferably from about 20 to 120 F. The cost of employing the adhesive strip can should be substantially no greater than the cost for a key-opening can, and this inherently requires that the strip be capable of being adhered to can blanks in a continuous operation, with the Abond being made in a matter of seconds or a split-second.

ACCOMPLISHMENTS OF THE INVENTION The present invention provides what we believe to be the first adhesive strip for hermetically sealing a metal can which is capable of withstanding the pressure of coffee stored in the can and yet may be opened at virtually any ambient temperature Without any sort of appliance by easily peeling the strip away with the fingertips. The coffee can consists of a metal container portion and a metal lid portion in abutting relation to provide a smooth continuous face extending substantially around the can. The line of abutment between the container and lid portions, whether circular or other shape, ordinarily lies in a single plane to which said face is perpendicular. The adhesive strip is adhered by its own adhesive directly to said face and extends at least 1A inch on each side of the line of abutment over its full length to seal the container and lid portions together.

In a slip-cover can, the line of abutment extends completely around the can so that the adhesive strip must overlap itself to provide a hermetic seal. Because of the discontinuity in the surface underlying the adhesive strip at the initial point of overlap and because of the discontinuity of the face of the can at the soldered seam, it is normally preferred that the slip-cover can be sealed with adhesive strips having relatively thick adhesive coatings. Ordinarily a thickness of about 3 to 5 mils is sufficient where the container portion is offset at the line at which it abuts the lid portion to provide a butt joint, but when there is no such offset, an adhesive thickness of about 8-10 mils may be necessary to assure a permanent hermetic seal.

In collar cans, the line of abutment is normally discontinuous for a short distance on either side of the soldered seam, over which distance the external surfaces of the container and lid portions are joined to each other and may be scored in continuation of the line of abutment. For such cans the adhesive thickness is desirably less as a cost saving and also to afford better resistance to creep, usually about 1/2 to 3 mils and preferably not more than 11/2 mils. After the adhesive strip is removed to separate the two portions of the can along the line of abutment, the lid portion is folded back. The can may be so designed that flexing of the lid separates it from the container portion along a scored line, as in present-day key-opening coffee cans, or so designed that the lid portion is permanently hinged to the container portion, as in key-opening shortening cans.

The use of the terms lid portion and container portion does not imply that one portion is smaller than the other. For example in canning a ham, it is desirable that the abutment line be near the center of the can for easy removal of the ham from the container portion.

The same adhesive strip may be used to seal dispensing openings in liquid-containing metal cans, and in spite of the effective hermetic seal, it is easily peeled away cleanly with the fingertips.

SUMMARY OF THE INVENTION The foregoing accomplishments are attained by the use of an adhesive strip which has a tough, strong, fiexible, nonstretchy backing member that is preferably a soft metal but may be a tough plastic such as polyethylene terephthalate which has been oriented by stretching 2.5 or more in at least the lengthwise direction. The backing member carries a smooth, dense, uniform coating of thermoplastic and heat-activatable adhesive, the adhesive coating being so selected to exhibit the following critical features:

(a) A contact angle as herein defined of 55-80,

(b) Continuous resistance to dead load shear of at least p.s.i. at 120 F. without creep, and

(c) Adhesion to clean tin plate in seconds under heat and pressure and a peel resistance therefrom at 75 F. of 21/2 to 61/2 pounds and preferably about 3 to 4 pounds per 1/z-inch width.

When so selected, the instantaneous peel resistance of the adhesive coating from the clean tin plate at any temperature from to 100 F. varies no more than i50% from the average at such temperature, and the average peel resistance at any such temperature varies no more than i% from the average peel resistance at 75 F.

The present invention does not utilize adhesives which are normally characterized as pressure-sensitive adhesives. Instead, the adhesive strip used in making the easyopening can of our invention employs a thermoplastic, heat-activatable adhesive which is essentially nontacky and is characterized by the unusual property that when the strip is adhered under heat and pressure by its own adhesive to clean tin plate, very slight but noticeably increased force is required to peel the strip away at increased rates up to at least 200 inches per minute. This characteristic is noted at all ordinary ambient temperatures. The slope of a curve of peel force vs. rate is positive at any temperature at least from 40 F. to 100 F. at rates of peel up to 200 or more inches per minute.

Preferably the adhesive coating is selected to afford continuous resistance without creep at temperatures up to 120 F. to dead load shear of at least 50 p.s.i. so that an adhesive strip of only about 3s-inch width will hermetically seal a standard-size coffee can against the pressure exerted at that temperature by the coffee vapors. Adhesives having a resistance to dead load shear of more than 50 p.s.i. at 120 F. permit the use of a narrower strip, while a wider strip makes adhesives of somewhat less resistance to dead load shear useful for purposes of this invention, 25 p.s.i. being sufficient for an adhesive strip of 3r-inch width centered over the line of abutment between the lid and container portions of the can or for narrower strips if the coffee is to `be distributed in areas where hot temperatures are not encountered.

Few adhesives afford the continuous resistance at F. to dead load shear of at least 25 p.s.i. that will keep coffee hermetically sealed with a narrow adhesive strip and also permit smooth and easy removal of the strip from clean tin plate so that the housewife is able to peel it away from the can with her fingertips. Those adhesives which have best satisfied this extraordinary combination of properties are characterized by high tensile strength coupled with high elongation. In general, the tensile strength exceeds 1200-1500 p.s.i. and the elongation at break exceeds 500%. Preferably the tensile strength is about 2000-4500 p.s.i. and the elongation about 500- 750% so that the modulus in tension is in the approximate range of Z50-900 p.s.i. These measurements may be made following ASTM test method D412-61T.

In general, the preferred adhesives are characterized by low brittle points, usually below 60 F.

When the adhesive strip of this invention is applied to other metals commonly used in making cans or is applied over lithographed coatings and finishing varnishes commonly used in decorative can labels, the peelback resistance continues to be very uniform at all ambient temperatures and remains remarkably close to values obtained on tin plate, generally well within 1 to 6 pounds for a %-inch strip.

To make the adhesive strip less subject to being opened prematurely, as by mischievous children, it may be desirable that somewhat higher force be required to begin peeling it back. This is readily accomplished by differential treatment of the face of the can underlying the tab portion of the adhesive strip. For example, the finishing varnish over a lithographed can may be omitted at the tab area. Where the adhesive strip is used to seal dispensing openings, the same effect is accomplished without such differential treatment by providing the adhesive strip with a greater effective Width at the tab than at the dispensing opening or openings.

The backing member should be 1 to 8 mils in thickness and should be capable of being pulled back upon itself without rupture. For convenience of removal of the adhesive strip, it should neither break nor elongate more than 25% under a tension of 4 pounds. The backing member should also have sufficient strength to withstand the force exerted at 120 F. by vapors in a can of coffee, that force being about 10 pounds per lineal inch atl the rim of the cover of a oneor two-pound can of the size presently used for coffee. To provide an adequate margin of safety, the backing material at a width of one inch should have a strength at break of at least 15 pounds.

A better understanding of the present invention may be gained by reference to the drawing in which:

FIG. 1 shows the adhesive strip material wound upon itself in a roll;

FIG. 2 shows a metal blank from which may be formed a collar can that embodies this invention;

FIG. 3 shows in perspective a collar can formed from the blank illustrated in FIG. 2;

FIG. 4 is an enlarged fragmentary section along line 4 4 of FIG. 3;

FIG. 5 shows in perspective a slip-cover can which is hermetically sealed by an adhesive strip in the practice of this invention;

FIG. 6 is an enlarged fragmentary section along line 6-6 of FIG. 5

FIG. 7 shows an end cap for a sanitary can wherein dispensing holes are sealed by an adhesive strip in the practice of this invention; and

FIG. 8 is an enlarged fragmentary section along line 8 8 of FIG. 7.

Referring first to FIG. 1, the illustrated roll 10 of adhesive strip material has a strong metal backing 11 carrying a coating of thermoplastic, heat-activatable adhesiye 12. For economy of manufacture, the roll may initially be 2 or 3 feet wide and subsequently slit into widths of 1A to 3A inch for application to individual cans or can blanks. Preferably the backing 11 carries on its surface 13 opposite the adhesive coating 12, a thin low-adhesion backsize coating to insure against blocking or transfer of the adhesive when the strip material is unwound from the roll. Effective low-adhesion backsizes are described in U.S. Pat. No. 2,532,011 and No. 2,607,711. The side of the backing 11 beneath the adhesive coating 12 may be treated to improve the anchorage of the adhesive and thereby insure that no adhesive transfers to the can when the adhesive strip is peeled olf. For example, the metal backing 11 may be etched to promote adhesion.

An adhesive strip 14 slit from the roll 10 and cut to suitable length may be applied to a can blank 15 to completely seal a long slit 16 as shown in FIG. 2. The ends of the slit 16 terminate in holes 17 which are designed to release the aroma of coffee vapors sealed in the can. From the holes 17 to the edges of the blank, scored lines 18 make it easy to separate the lid from the container portion of the can after the strip 14 is peeled away. At one end, the strip 14 is folded back on itself to provide a tab 19 which may be grasped With the fingers to begin peeling the strip.

After the adhesive strip 14 is firmly adhered in place, the can blank 15 may be formed into the can with a seam 22 as shown in FIG. 3. Then a collar 24 is fastened to the inside of the can opposite the strip 14, as shown in FIG. 4, so that the can closes tightly when the lid portion 20 is replaced over the collar on the container portion 21 after the adhesive strip 14 is removed. Then one end cap is applied (usually the top 23), the can filled with coffee, and the other end cap completes the hermetic seal. These general can-forming procedures are well known and form no part of our invention apart from the novel adhesive Strip and its usage in the manner and combination as herein described.

In FIG. 5 there is shown a slip-cover can which is sealed with an adhesive strip 28 extending completely around the can to cover the whole line 29 at which the lower edge 30 of the lid portion 31 abuts the container portion 33, illustrated in more detail in cross-section by FIG. 6. The lid portion 31 and container portion 33 are recessed slightly as indicated at 34 and 3S respectively so that the adhesive strip 28 does not protrude beyond the major exterior surface of the can, thereby protecting the strip from being accidentally caught on something during handling and shipment. Within the recess between 34 and 35, the lid portion 31 and container portion 33 together provide a smooth face against which the adhesive strip 28 can be laid flat and adhered without contortion.

In order that the slip-cover can illustrated in FIG. 5 may be hermetically sealed, the adhesive strip 28 overlaps itself beneath the pull tab 36. To assure a good hermetic seal at this overlap, it is preferred that the adhesive coating have a thickness at least equal to the thickness of the backing member of the adhesive strip. If this preference is met, the adhesive coating will normally have adequate thickness to maintain a reliable hermetic seal at the soldered seam 37.

FIG. 7 shows an end vcap 40 for a sanitary can from which liquids may be dispensed through a preformed opening 41 after removal of an adhesive strip 42 which hermetically seals the opening. As seen in FIG. 8, the adhesive strip 42 comprises a plastic film backing 43 carrying a thermoplastic, heat-activatable adhesive coating 44 which bonds the adhesive strip to the end cap. At one end of the adhesive strip 42 has been applied a piece of pressure-sensitive adhesive tape 46 to prevent the adhesive coating 44 from adhering to the end cap at the pull tab 45. The backing 47 of the tape 46 may be paper to permit a better grip of the tab.

Example 1 In a specific embodiment, adhesive strip material of this invention was made using as its backing 6-mil, dead-soft 5052 aluminum foil which has a strength at break of about 170 pounds per inch of width. When slit to widths of 1A to 1% inch, it neither breaks nor elongates more than 1% under a tension of four pounds. The side of the foil to which the adhesive was to be applied was first coated with a very thin adhesion-promoting primer having a dry weight of about one grain per 24 square inches. The particular primer was the vinyl resin VMCH which is a copolymer of 86 parts vinyl chloride, 13 parts vinyl acetate and 1 part maleic anhydride.

Over the dried primer layer was applied an adhesive layer by coating a 25% by weight solution in methyl ethyl ketone of an adhesive composition consisting of 70 parts by weight of nitrile rubber, 30 parts of a high molecular weight copolymer of equal parts by weight of bisphenol A and epichlorohydrin, and 50` parts of the vinyl resin VAGFL The thickness of the adhesive coating was about one mil after drying for three minutes at 260 F. The nitrile rubber, which has a monomer proportion of about l65 parts butadiene and 35 parts acrylonitrile and a Mooney viscosity (ML 2 212 F.) of 85-112, is marketed as Chemigum N-S. The bisphenol-epichlorohydrin copolymer, which has an average molecular Weight within the approximate range of 20,000 to 30,000 and a glass transition temperature of about C., was marketed as Phenoxy 8 (now Phenoxy PAHI). The vinyl resin VAGH is a partially hydrolyzed copolymer of about 91 parts Vinyl chloride and 9 parts vinyl acetate.

In both one-pound and two-pound coffee cans of the size in present general use in the United States, the coffee vapors in a can held at F. exert a force of about 10 pounds per lineal inch at the rim of the cover. Hence, in coffee cans constructed in accordance with FIG. 3 of the drawing, an adhesive strip of %inch width which is centered over the line of abutment between the container portion and the lid portion, and thus has 3716 square inch holding area on each portion per inch of circumference, would be subjected to a continuous load in shear at 120 F. of roughly 50 pounds per square inch. When a layer of adhesive of this example one mil in thickness was tested at 120 F., it resisted a dead load in shear of 60 pounds per square inch without discernible movement for three weeks, at which point the test was discontinued. The adhesive failed in this test at 70 pounds per square inch. This provides a margin of safety for use of the adhesive strip material of this example at a width of 3% inch for sealing coffee cans.

The adhesive had a tensile strength of 1500 p.s.i., an elongation at break of 1000% and a modulus in tension of p.s.i. (ASTM method D412-61T).

The adhesive strip material was slit to a width of inch and then bonded to clean tin plate by first passing the strip and plate under infrared lights to heat the adhesive to about 350 F. and then between a pair of metal cylinders, the surface temperature of which was 350 F. The strip and plate were advanced at a rate of about 100 feet per minute, were subjected to a force of about 80 pounds per lineal inch by the cylinders, and then allowed to cool to room temperature.

The adhesion of the %-inch adhesive strip to the tin plate was tested at various temperatures for resistance to 180 peelback at a rate of 20 inches per minute (i.e., the rate of movement of the free end of the strip relative to the adhered portion). The following average values in pounds for 3s-inch width were obtained- At R): Pounds -40 4 3%. 40 3 75 3 120` 21/2 Such a small difference over this wide a temperature range in average peelback adhesion values of a thermoplastic, heat-activatable adhesive has been noted by us only with adhesives which are useful in the present invention. At each temperature, the instantaneous resistance to peelback for each test strip did not vary from the aboverecorded average at that temperature more than i20% during the test. There was never any transfer of adhesive to the tin plate.

This Xs-inch adhesive strip has been employed in making can blanks of the type illustrated in FIG. 2 of the drawing which were formed to cylindrical shape and hermetically sealed by application of end caps. In some of the can blanks, the adhesive strip was applied to clean tin plate and in others it was applied over lithographed coatings of the type commonly used in decorative labels for coffee cans. In all cases the cans easily withstood the dead load forces which coffee vapors exert at 120 F. in standard-size oneand two-pound cans. Such tests indicated that the cans could be used to package a variety of foods and would remain hermetically sealed indefinitely under all normal conditions of storage and handling.

To open the cans, it was necessary only to grasp the folded tab at one end of the adhesive strip lightly between the thumb and forelinger, and the strip was easily and smoothly peeled away. No residue of adhesive was observable on any of the cans.

Example 2 Adhesive strip material was made using a different adhesive coated to a one-mil dried thickness on S-mil, dead-soft 3003 aluminum foil primed as in Example 1. The adhesive was a mixture of equal parts of the same nitrile rubber and the bisphenol-epichlorohydrin c0- polymer used in the adhesive of Example 1. This adhesive layer resisted dead load shear of 60 p.s.i. at 120 F. for three Weeks 'without observable creep, but failed at 70 p.s.i.

The adhesive strip material of this example was slit to strips of a width of inch which were adhered in the same manner as in Example l to clean tin plate and tested at various temperatures for resistance to peelback at 180 with these average results- At E): Pounds -40 5 0 5%, 40 3 75 3 At any temperature in the test, the instantaneous resistance to peelback of each adhesive strip tested did not vary more than 120% from the above-recorded average at such temperature. There was never any observable transfer of adhesive to the tin plate.

Adhesive strips of the same width were used in making collar cans of the type shown in FIG. 3 of the drawing, and tests showed these to be useful for canning coffee.

The same adhesive and backing material were used to make adhesive strip material wherein the adhesive coating and backing were each 3 mils in thickness. This was slit to a width of -1/2 inch and used in sealing slip-cover cans as illustrated in FIG. 4 of the drawing. Good hermetic seals were obtained in spite of the discontinuity in the face of the can at the soldered seam and the necessity 'of overlapping the strips. The 3-mil aluminum foil backing of this example at 1/2 inch width has a strength at break of about 50 pounds.

Example 3 Another adhesive strip material was prepared in the same Amanner as in Example 1 except with different materials. The backing was 5-mil, dead-soft 3003 aluminum foil. VMCH was again employed as the adhesion-promoting primer at a dry weight of about one grain per 24 square inches.

The adhesive coating, applied using a 15% solids solution in methyl ethyl ketone, was a mixture of 90 parts by weight of a polyester-based urethane polymer having essentially no reactive sites and l0 parts of the vinyl resin VAGH. The polyesterurethane polymer was obtained commercially under the designation Estane 5740X2. Analysis indicated that it was prepared from about 12 mols of diphenyl methane diisocyanate and 13 mols of an adipate polyester of about 820 molecular weight and that the average molecular weight of the urethane polymer was about 14,000. This adhesive when freshly prepared has a tensile strength of 2050 p.s.i., elongation at break of 660%, and modulus in tension of 310 p.s.i. After 2-3 weeks its tensile strength increases to about 4000 p.s.i. and eventually reaches an ultimate Value of about 5300 p.s.i.

A one-mil coating of this adhesive on the primed aluminum foil exhibited a resistance to dead load in shear of 60 pounds per square inch at 120 F. for three weeks without failure but failed at pounds per square inch.

A 3/s-inch strip of the adhesive strip material of this example was adhered to clean tin plate in the same way as in Example 1 and tested for resistance to 180 peelback with the following average results- At R): Pounds -40 31/2 0 3 40 3 21/2 21/2 With each test strip, the instantaneous resistance to peelback at any test temperature did not vary more than i20% from the average at such temperature. There was never any transfer of adhesive to the tin plate.

This 3s-inch adhesive strip was used in making tin plate can blanks of the type shown in FIG. 2 of the drawing which were formed into cylindrical cans. Tests of the cans indicated their utility for packaging coffee or other perishable commodities under lasting hermetic seal, with the strip providing an easy-opening feature.

Adhesive strip materials identical to that of this example except for the omission of the VAGH vinyl resin from the adhesive coating or except for increase in proportion of the vinyl resin up to about 60 parts per 100 parts by weight of the polyesterurethane polymer have demonstrated utility in the practice of this invention. At substantially greater proportions of the -vinyl resin, the adhesive strip does not peel away in the smooth, easy manner by which the adhesive strips of this example are characterized.

Example 4 Over a period of time, it was found that adhesive strip material made in accordance with Example 3 would vary from lot to lot in its adhesion to tin plate. Eventually this was traced to small amounts of calcium stearate included as a dusting powder in the Estane 5740X2 polyesterurethane polymer as supplied by the manufacturer (which has since redesignated that product as Estane 5702). To test the effect of the calcium stearate, the same polymer Without calcium stearate or other dusting powder was obtained under the designation Estane 5703 and was mixed in equal parts with Estane 5701 (formerly Estane S740X1) which chemically is basically the same as Estane 5703 and has no calcium stearate but has slightly higher tensile strength, i.e., a reported 8900 p.s.i. versus the 5300 p.s.i. of Estane 5703. The Estane 5701 was employed in order to achieve somewhat better high temperature performance. Since Estane 5701 is insoluble in methyl ethyl ketone, dimethylformamide solvent was employed for coating the mixture on the primed aluminum foil.

Proportion of calcium stearate Bonding temperature F.) 0 0.05% 0. 10% 0.20%

4. 1-5. 7 3. 2-4. 6 3. 2-4. 2 2. 6-3. 4 3. 95. 9 2. 8-3. 4 2. 2-3. 0 2. 0-2. 5 3. 4-4. 4 (l) 2. 2-2. 9 2. 0-2. 5 S10-320 2. 4-3. 1 (l) 1. 9-2. 3 (l) 1 N Ot tested.

As compared to these laboratory results, production on a continuous basis results in somewhat lower peel values so that at a calcium stearate content of 0.10% or more, it would be necessary to call for a bonding temperature of about 370-380 F. Because such a narrow temperature range cannot be maintained in production on a reasonable basis, the eventual result would be that at times the bonds would be undesirably weak (when the temperature drops below 360 F.) and at times there would be danger of degradation (at temperatures of 390 or more). By using a calcium stearate content of 0.05%, a much wider, more reasonable temperature range of at least 350-380 F. can be used and in the absence of calcium stearate, a reasonably broad temperature range vthe angle between the edge of the drop and the adhesive surface beneath the drop is measured after 15 seconds. In this test, the following average results were obtained vwith the adhesives of this example:

Calcium stearato content, percent- Contact angle (deg.) 60

From these and tests on other adhesives, it was found 'that at contact angles above about 80, adhesive strips made and bonded in large-scale production provide un- -desirably erratic or low peelback adhesion from metal surfaces commonly used in canning. On the other hand,

contact angles below 55 indicate unduly erratic or high 'peelback adhesion values.

Example 5 Adhesive strip material of this invention has been made using as the backing unprimed 5-mil polyethylene terephthalate film which had been biaxially oriented about 3 in each direction. Such iilm when slit to 1/z-inch width elongates 1% to 2% and does not break under a tension of 4 pounds.

The adhesive coating of the adhesive strip material was a copolymer of about 72 parts ethylene and 28 parts v vinyl acetate which is commercially sold as Elvax 250. This adhesive has a tensile strength of about 2000 p.s.i.,

and an elongation at break of about 750%, a modulus in tension of about 270 p.s.i. and a melt index of 12-18. The adhesive was heated to 350 F. and extruded directly onto the polyethylene terephthalate film to provide an adhesive coating about one mil in thickness. The adhesive coating exhibited at 120 F. a resistance to dead load shear of 50 pounds per square inch for three weeks without discernible movement. It failed at 60 pounds per square inch.

This adhesive strip material was slit to 3s-inch width and adhered to clean tin plate following the proceduers of Example 1 except using temperatures of 260 F. and slowing the rate to 50 feet per minute. Tests for resistance to 180 peelback showed average values- At F.: Pounds Each adhesive strip in the test exhibited smooth, uniform removal from the tin plate, never varying at any test temperature from the average removal resistance of that strip at such temperature more than 120%.

Collar cans of the type illustrated in FIG. 3 of the drawing made with the 3s-inch adhesive strip were found to be useful for hermetically sealing perishable foods such as coffee and were easily opened by peeling the strip away with the lingers.

The adhesive strip material of this example was varied to the extent that the thickness of the biaxially-oriented polyethylene terephthalate ilm backing and of the adhesive coating were each 3 mils, whereupon a satisfactory hermetic seal was obtained on a slip-cover can as illustrated in FIG. 4 of the drawing.

Example 6 The adhesive strip of this example employed 3-mil polyethylene terephthalate lilm which had been biaxiallyoriented about 3 in both directions and coated with aluminum vapor to reduce permeability to oxygen and to opacify the film. The adhesive was a copolymer of about 72 parts of ethylene and 28 parts of vinyl acetate (Elvax 2160) which differed from the copolymer of Example 5 in that its melt index was about 3 and it had an elongation at break of about 850%.

The copolymer was heated to 410 F. and extruded directly onto the uncoated surface of the polyester film to provide an adhesive coating about 4 mils in thickness, and this was followed by irradiation through the adhehesive coating by the procedure of Charbonneau-Abere Pat. No. 3,188,266 to improve the bond between the adhesive and backing. The contact angle for this adhesive was 73-78.5 (average 74.5). The adhesive coating resisted a dead load shear of 45 p.s.i. for 64 hours at F. without discernible movement, at which point the temperature was increased to F. and one of the two test specimens failed at 42 hours. The temperature was then increased to F. and the other specimen failed at 2.3 hours.

This adhesive strip material at 'V2-inch widths was adhered to clean tin plate in a heated platen press at about 325 F. under about 30 p.s.i. for about 5 seconds. Resistance to 180 peelback at 75 F. and 11/2 to 2 pounds,` an undesirably low value. When the platens were heated to about 375 F., the peelback adhesion was 3 to 5 pounds per 1/z-inch width.

In order to permit the use of a lower bonding temperature, adhesive strip material of this example was exposed to mild corona treatment in an apparatus including a rotatable 20-inch aluminum drum which was electrically connected to ground. Concentrically located Ms inch from the surface of the drum were two aluminum shrouds, each extending 12 inches along the surface of the drum and each covered with electrical insulation to prevent arcing. To each shroud was applied a direct current of 7400 volts pulsating at 400 cycles per second. With the strip material moving around the drum at 36 feet per minute and with the adhesive surface facing the shrouds, the electrical discharge modified the properties of the adhesive surface. Its contact angle now was 65- 69 (average 67). When bonded in a heated platen press at about 325 F. and 30 p.s.i. for 5 seconds, its peelback at 75 F. was 4 to 6.5 pounds per 1/2-inch width. When the bond was formed at 375 F., the peelback adhesion was 7 Ito lO pounds per 1/z-inch width.

Another portion of the adhesive strip material was given a stronger corona treatment with the voltage at about 9100 volts, the drum continuously wet with water and the speed et feet per minute. The treated adhesive surface had a contact angle of 43.5 to 50.5 (average 47.5). When bonded at about 325 F. and 30 p.s.i. for 5 seconds, the peelback adhesion at 75 F. was 7 to 8 pounds per 1/2-inch width. Lower peelback adhesion would have been obtained at lower :bonding temperatures, but not over a suitably wide range of ternperatures for large-scale production.

From this and Example 4, it is seen that the contact angle is valuable for screening out Iunsuitable adhesive coatings for `quality control of a selected adhesive coating. Within the range of contact angles from 55-80, it has always been possible to nd a range of useful bonding temperatures of sufficient breadth for large-scale production.

Example 7 Adhesive strip material was made as in Example 6 except that the adhesive was a copolymer of about 80 parts ethylene and parts vinyl acetate (Union Carbides DQDB 1867) having a melt index of about 2, a tensile strength of about 3500 p.s.i. and a softening point of about 320 F. The copolymer ywas extruded at 375 F. to provide an adhesive coating 4 mils in thickness on the polyester film followed `by irradiation through the adhesive coating by the procedure of Patent No. 3,188,266. The adhesive surface was then given the mild corona treatment described in Example 6.

. These adhesive strips at l-inch and 'Vs-inch widths were employed in the manner illustrated in FIGS. 7 and 8 for the canning of fruit ljuice. The adhesive strips were sealed at 255 F. under 140 p.s.i. for one second. The juice was then filled at a temperature of 205-2l1 F. into the open bottom of each can. After the bottom end cap was seamed onto the can and upon cooling in water to room temperature, the pressure in the can changed from positive to l7-20 inches of vacuum. All 110 cans in the run were effectively sealed and showed evidence of vacuum in the can, and the cans as a whole satisfactorily withstood aging tests and were easily opened by peeling back the adhesive strips.

Other film :backings have demonstrated utility in the adhesive strip material of this invention. Among these was a film of the bisphenol-epichlorohydrin copolymer described in Example 1 (Phenoxy 8) which had been stretch-oriented in the lengthwise direction to a thickness of 4 mils. This film has a strength at break of about 35 pounds per inch. The stretch-oriented polycarbonate film Lexan is useful where the adhesive is applied as a hotmelt coating or vby transfer coating, although it exhibited a tendency to craze in contact with solvents from which many adhesives are coated. Rigid polyvinyl chloride film is another particularly suitable film backing.

We claim:

1. For use in a metal can as an easy-opening feature, an adhesive strip which hermetically seals the can but is easily peeled away with the fingertips, said adhesive strip comprising:

(l) a tough, flexible, nonstretchy plastic or soft metal tape backing member at least about 1A inch in width and about 1 to 8 mils in thickness which neither breaks nor elongates more than under a tension 12 of 4 pounds and is capable of being pulled back upon itself without rupture, said backing member carrying (2) a smooth, dense, `uniform coating of thermoplastic,

heat-activatable adhesive of about 1/2 to l0 mils in thickness, which adhesive coating comprises a polyurethane rubber, a mixture of nitrile rubber and a high molecular weight bisphenol-epichlorohydrin copolymer, or a copolymer of a major proportion of ethylene and a minor proportion of vinyl acetate and exhibits (a) a contact angle as herein defined of 55-80", (b) continuous resistance to dead load shear of at least 25 p.s.i. at 120 F. `without creep, and (c) adhesion to clean tin plate in seconds under heat and pressure over a range of useful bonding temperatures of sufficient breadth for largescale production and a peel resistance therefrom of 21/2 to 61/2 pounds per 1/z-inch width at 75 F., the peel resistance at any temperature from 40 to 100 F. varying no more than 160% from the average at such temperature, and the average peel resistance at any such temperature varying no more than i50% from the average peel resistance at 75 F.

2. For use in a hermetically sealed can suitable for the storage of coffee at temperatures at least as high as F., which can consists of a metal container portion and a metal lid portion in abutting relation to provide a smooth continuous face extending substantially around the can and on each side of the line of abutment, an adhesive strip adherable by its own adhesive directly to said face for releasably sealing said container and lid portions together, which strip is then readily peelable -by hand from the can at any temperature from 40 F. to 75 F. without transfer of adhesive to the can to provide easy and convenient separation of the lid portion from the container portion, said adhesive strip comprising (1) a tough, fiexible nonstretchy plastic or soft metal tape backing member about 1A to 1% inch in width and 1 to 8 mils in thickness which neither breaks nor elongates more than 25% under a tension of 4 pounds and is capable of being pulled back upon itself without rupture, said backing member carrying (2) a smooth, dense, uniform coating of rubbery, thermoplastic heat-activatable adhesive of about 1/2 to 5 mils in thickness, which adhesive coating comprises a polyurethane rubber, a mixture of nitrile rubber and a high molecular weight bisphenol-epichlorohydrin copolymer, or a copolymer of a major proportion of ethylene and a minor proportion of vinyl acetate and exhibits (a) a contact angle as herein defined of 55-80, (b) continuous resistance to dead load shear of at least 25 p.s.i. at 120 F. without creep, and (c) adhesion -to clean tin plate n seconds under heat and pressure over a range of useful bonding temperature of sufficient breadth for largescale production and a peel resistance therefrom of 21/2 to 61/2 pounds per 1/z-inch width at 75 F., the peel resistance at any temperature from 40 to 100 F., varying no more than 150% from the average at such temperature, and the average peel resistance at any such temperature varying no more than 50% from the average pecl resistance at 75 F., such peel resistance from clean tin plate approximating that of the adhesive strip from decoratively lithographed can stock.

3. ln a hermetically sealed can containing coffee or other perishable commodity, which can withstands internal pressures from vapors of coffee stored in the can at 120 F. and consists of a metal container portion, a metal lid portion in butt joint relation to provide a smooth continuous face extending substantially around the can and 0n each side of the line of abutment, and

an adhesive strip adhered by its own adhesive directly to said face and releasably sealing said container and lid portions together, said can being characterized by the fact that the adhesive strip is readily peelable by hand from the can at any temperature from -40 F. to 75 F. without transfer of adhesive to the can to provide easy and convenient separation of the lid portion from the container portion and comprises (1) a soft metal tape backing member about 3A; to

`1/2 inch in Width and about 3 to 5 mils in thickness which neither breaks nor elongates more than 25% under a tension of 4 pounds and is capable of being pulled back upon itself without rupture, said backing member carrying (2) a smooth, dense, -uniform coating of rubbery,

thermoplastic heat-activatable adhesive of about 1/2 to mils in thickness, and having a tensile strength of at least 2000 p.s.i. and an elongation at break of about 500 to 1000%, which adhesive coating comprises a polyurethane rubber, a mixture of nitrile rubber and a high molecular Weight bisphenolepichlorohydrn copolymer, or a copolymer of a major proportion of ethylene and a minor proportion of vinyl acetate and exhibits:

(a) a contact angle as herein defined of 55-80", (b) continuous resistance to dead load shear of at least 25 p.s.i. at 120 F. without creep, and (c) adhesion to clean tin plate in seconds under heat and pressure over a range of useful bonding temperatures of sufcient breadth for largescale production and a peel resistance therefrom of 21/2 to 61/2 pounds per 1/z-inch width at 75 F., the peel resistance at any temperature from 40 to 100 F. varying no more than 50% from the average at such temperature, and the average peel resistance at any such temperature varying no more than i50% from the average peel resistance at 75 F., such peel resistance from clean tin plate approximating that of the adhesive strip from decoratively lithographed can stock.

4. In a hemetically sealed can for liquids, one end cap of which is formed with a dispensing opening across which an adhesive strip is adhered by its own adhesive directly to said end cap to seal the opening, which strip is readily peelable by hand from the can at any temperature from 40 F. to 75 F. without transfer of adhesive to the can to provide easy and convenient exposure of said opening, said adhesive strip comprising:

( 1) a tough, flexible, nonstretchy plastic or soft metal tape backing member at least about 1A inch in width and about 1 to 8 mils in thickness which neither breaks nor elongates more than 25% under a tension of 4 pounds and is capable of being pulled back upon itself without rupture, said backing member carrying (Z) a smooth, dense, uniform coating of thermoplastic, heat-activatable adhesive of about 1 to 5 mils in thickness, which adhesive coating comprises a polyurethane rubber, a mixture of nitrile rubber' and a high molecular Weight bisphenol-epichlorohydrin copolymer, or a copolymer of a major proportion of ethylene and a minor proportion of vinyl acetate and exhibits (a) a contact angle as herein delined of 55-80, (b) continuous resistance to dead load shear of at least 25 p.s.i. at 120 F. without creep, and (c) adhesion to clean tin plate in seconds under heat and pressure over a range of useful bonding temperatures of suicient breadth for largescale production and a peel resistance therefrom of 21/2 to 61/2 pounds per 1/z-inch Width at F., the peel resistance at any temperatnre from 40 to 100 F. varying no more than i50% from the average at such temperature, and the average peel resistance at any such temperature Varying no more than i50% from the average peel resistance at 75 F.

5. For use in a hermetically sealed can as defined in claim 1, an adhesive strip having a polyurethane rubber adhesive coating comprising a mixture of parts by weight of a polyesterurethane polymer and 10 parts of a partially hydrolyzed copolymer of about 91 parts vinyl chloride and 9 parts vinyl acetate.

6. For use in a hermetically sealed can as defined in claim 1, an adhesive strip having an adhesive coating comprising a mixture of 50 parts by weight of nitrile rubber, 50 parts of a high molecular weight bisphenolepichlorohydrin copolymer.

7. For use in a hermeticaily sealed can as defined in claim 1, an adhesive strip having an adhesive coating comprising a copolymer of 72-80 parts by Weight of ethylene and 20-28 parts of vinyl acetate.

References Cited UNITED STATES PATENTS 2,467,875 4/ 1949 Andrews 99-178 2,576,148 11/ 1951 Schechtman 117-122 2,629,534 2/ 1953 Reynolds 229-35 2,850,406 9/1958 Gold 117-122 3,013,914 12/1961 Willard 154-43 3,043,716 7/1962 Busse et al. 117-132 3,156,659 11/ 1964 Robitschek 260-2.5 3,186,581 6/ 1965 Schneider et al. 220-53 3,215,678 11/1965 Adelman 260-80.5 3,231,078 l/1966 Balocca et al. 206-46 3,244,306 4/ 1966 Stolk 215-40 WILLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant Examiner U.S. C1. X.R. 

